Lubricating oils used in gasoline and diesel crankcases comprise a natural and/or synthetic basestock containing one or more additives to impart desired characteristics to the lubricant. Such additives typically include ashless dispersant, metal detergent, antioxidant and antiwear components, which may be combined in a package, sometimes referred to as a detergent inhibitor (or Dl) package. The additives in such a package may include functionalised polymers but these have relatively short chains, typically having a number average molecular weight M.sub.n of not not more than 7000.
Multigrade oils usually also contain one or more viscosity modifiers (VM) which are longer chain polymers, which may be functionalised to provide other properties when they are known as multifunctional VMs (or MFVMs), but primarily act to improve the viscosity characteristics of the oil over the operating range. Thus the VM acts to increase viscosity at high temperature to provide more protection to the engine at high speeds, without unduly increasing viscosity at low temperatures which would otherwise make starting a cold engine difficult. High temperature performance is usually measured in terms of the kinematic viscosity (kV) at 100.degree. C. (ASTM D445), while low temperature performance is measured in terms of cold cranking simulator (CCS) viscosity (ASTM D5293, which is a revision of ASTM D2602).
Viscosity grades are defined by the SAE Classification system according to these two temperature measurements. SAE J300 defines the following grades:
______________________________________ Maximum CCS kV 100.degree. C. kV 100.degree. C. SAE viscosity Viscosity mm.sup.2 /s mm.sup.2 /s grade 10.sup.-3 Pa.s @ (.degree.C.) minimum maximum ______________________________________ 5 W 3500 (-25) 3.8 -- 10 W 3500 (-20) 4.1 -- 15 W 3500 (-15) 5.6 -- 20 W 4500 (-10) 5.6 -- 25 W 6000 (-5) 9.3 -- 20 -- 5.6 &lt;9.3 30 -- 9.3 &lt;12.5 40 -- 12.5 &lt;16.3 50 -- 16.3 &lt;21.9 ______________________________________
Multigrade oils meet the requirements of both low temperature and high temperature perfomance, and are thus identified by reference to both relevant grades. For example, a 5W30 multigrade oil has viscosity characteristics that satisfy both the 5W and the 30 viscosity grade requirements--i.e. a maximum CCS viscosity of 3500.10.sup.-3 Pa.s at -25.degree. C., a minimum kV100.degree. C. of 9.3 mm.sup.2 /s and a maximum kV100.degree. C. of &lt;12.5 mm.sup.2 /s.
Viscosity modifiers comprise polymers having an M.sub.n of at least 20,000. For ease of handling viscosity modifiers are usually employed as oil solutions of such polymers. When used in engines, oils are subjected to high mechanical shear, for example in bearings, pumps and gears, or to chemical attack such as oxidation, and the longer polymer chains of viscosity modifiers are broken which reduces their contribution to viscosity performance.
Shear stability is a measure of the ability of an oil to resist permanent viscosity loss under high shear--the more shear stable an oil, the smaller the viscosity loss when subjected to shear. Polymeric viscosity modifiers which make a significant contribution to kV100.degree. C. are not completely shear stable.
Shear stability of viscosity modifiers or oils containing them may be measured by a number of methods including the Kurt-Orbahn Diesel Fuel Injector test (CEC-L-14-A-88). Oil shear stability is quoted as the % loss of kV100.degree. C. of the oil in the test. VM shear stability is quoted as the shear stability index or SSI of the VM. SSI is the loss of kV100.degree. C. in the test by a 14 mm.sup.2 /s solution of the VM in a 5 mm.sup.2 /s diluent oil, the loss being expressed as a % of the kV100.degree. C. contribution of the unsheared VM polymer. The kV100.degree. C. contribution of the unsheared VM polymer can be determined by comparing the kV100.degree. C. of diluent oil with and without the polymer present. Thus: EQU SSI=(.eta..sub.i -.eta..sub.f)/(.eta..sub.i -.eta..sub.o)100,
where .eta..sub.i is the viscosity of the solution of VM in diluent oil, .eta..sub.o is the viscosity of the diluent oil without VM, and .eta..sub.f is the viscosity of the sheared VM solution.
Specifications for lubricants may be set in terms of a maximum loss of viscosity and/or minimum limit on after shear viscosity. The most severe requirements for oil shear stability at present are for oils that meet the VW500.00 specification and proposed ACEA specification, which require the kV100.degree. C. of the oil to be in grade (according to SAE J300) at the end of the shear test and to suffer a kV100.degree. C. viscosity loss not exceeding 15% in the Kurt-Orbahn Diesel Fuel Injector test. Thus for a multigrade oil meeting the 40 grade requirement of SAE J300 (e.g. a 15W/40 or 10W/40 oil) the oil must have a minimum kV100.degree. C. of 12.5 mm.sup.2 /s at the end of the test and a maximum kV100.degree. C. viscosity loss of 15%.
Economic VMs such as olefin copolymers have poor shear stability (high SSI). VMs with low SSI tend to be expensive. Shorter chain polymers which are used in functionalised form as dispersants are much more shear stable but make only a small contribution to kV100.degree. C. Thus the contribution to kV100.degree. C. made by the polyisobutenyl succinimide dispersants described for example in U.S. Pat. No. 4,234,435 is limited. In addition, attempts to increase viscosity contribution of conventional dispersants by increasing the treat rate can lead to problems with seal compatability and low temperature viscosity performance, which if combatted by lighter basestocks results in loss of diesel performance.
Thus conventional multigrade oils are not mechanically shear stable, and the presence of VMs increases cost and complexity of blending. VMs themselves also tend to have a detrimental effect on piston deposits, particularly in diesel engines, and on turbocharger intercooler deposits, particularly in the MTU test.